The present invention relates to zero-current detectors for high voltage DC power transmission lines and in particular to such a detector employing a saturable magnetic reactor.
The sensing of current (including zero current) in an AC line is typically done with a "current transformer" which consists of a large iron core which circles the AC line and is dielectrically insulated from it. If a multi-turn winding on the iron core is loaded with a sufficiently small load resistance, then the voltage generated across the load resistance is a faithful replica of the current in the AC line.
For high voltage direct-current transmission, the magnetic flux surrounding the conductor is relatively constant so a conventional iron-core current transformer would not work, i.e., the core would merely saturate. If one omitted the iron core, i.e., merely used the voltage output of a coil adjacent to the line, this also is incapable of indicating the magnitude of the DC current. The reason for this is that the coil output voltage merely indicates the time-rate-of-change of the line current, i.e., the coil output would be identical for a current change going from 2,000 amp to 1,500 amp (in a given time) as for a current change going from 500 amp to zero amp in the same time. Thus, coil output would merely indicate a change in current, but not the magnitude of the current.
The desirability of providing zero-current sensors for high voltage DC power transmission lines is becoming greater because of the increased use of DC transmission lines for long distance power transmission. Any inefficiencies which may result in converting the alternating current to direct current and back again at the terminal ends of the transmission line are compensated for by the increased transmission efficiency of direct-current power. To provide increased system reliability, the DC power transmission lines are typically provided in side-by-side pairs, so that if there is a failure in one pair, service may be restored through the system through the other pair. The use of such pairs is particularly prevalent along those portions of the transmission line where repair is difficult or would be slow, for example, along an underwater path. If a failure occurs on one of the transmission line pairs, it is highly desirable that the DC power be rapidly switched to the alternate line in order to minimize the disruption of service. This is done by gradually lowering the DC current on the line until the current is low enough so that the switches employed can interrupt it. The switches now employed cannot interrupt a DC current of more than approximately 10 amperes. Hence, it is necessary to locate a zero-current sensor at each switch to detect when the line current is low enough so that the switch can be opened without damage.
Typical high voltage DC power transmission lines operate at power levels of approximately 400,000 volts and 2,000 amperes. For such lines, present zero-current sensors must not only be placed near the terminal ends of the line but also along the lines at switch positions which may be located tens or hundred of miles from the terminal equipment. Each of these zero-current sensors disposed along the transmission line now comprises a free standing device called a "transductor", typically costing approximately sixty thousand dollars if rated at 400 kV. The primary reason for this high cost is the insulation required. Accordingly, the cost of such sensors significantly adds to the ultimate price of the DC power system.
U.S. Pat. No. 4,087,701 issued May 1978 to John M. Anderson and assigned to the same assignee as the present invention describes a transformer cascade for delivering relatively low levels of electric power to electronic systems and instrumentation operating on high voltage transmission lines. These transmission lines may be either direct current lines or alternating current transmission lines. In the Anderson invention, power is transmitted from a source at ground potential to instrumentation circuits operating at line potential through a cascade of transformers, each having relatively low voltage insulation and a turns ratio of approximately 1:1. The contents of this Anderson patent are hereby incorporated herein by reference.